This invention is in the field of power and impedance conversion technology. It represents an improvement in size, weight and protection features for AC transformers used today mostly for industrial applications. In this context, an AC transformer is defined as an inherently bi-directional coupler of AC electrical energy i.e. AC<->AC unlike the more common AC->DC->AC or DC->AC conversion process used for example in Uninterruptible power supplies and Sine-Wave inverters respectively.
Since the 1980's most traditional AC transformer applications e.g. Computer power supply, Television power supplies have moved to switch-mode power supply (SMPS) conversion methods which replace the bulky iron-core line-frequency transformer core with an AC->DC stage (rectifier+smoothing capacitor)+isolated DC->DC stage incorporating high frequency ferrite magnetics.
While these new technologies serve their applications very well, there is still a large market for traditional AC power transformers especially in the industrial field.
Although much heavier than high-frequency switch-mode AC->DC or AC->DC->AC power supply, the traditional line-frequency (50, 60 or 400 Hz) AC transformer remains largely untouched in industrial power applications due to the following reasons:                Low cost        Simple—no electronic components.        Extremely long life—no electrolytic capacitors, no semiconductors, no cooling fans needed.        Rugged—Withstand short term overloads and transients.        Zero electromagnetic interference.        
Of all the advantages to the industrial user, the ruggedness of a well-designed power transformer when installed in a power network cannot be matched with prior-art electronic circuitry.
Not only does this reliability stem from the inherent simplicity of a transformer, it is just as much a function of another inherent ability of the AC transformer i.e. to pass power and transients bi-directionally between AC source and the load.
A typical example illustrates this feature: Where high power inductive load is switched off quickly, there is often a large ‘kick-back’ of electrical transient energy towards the power supply.
While a switch-mode power supply with potentially an AC->DC->AC conversion system has no inherent way to deal with this surge coming back into its output (except to try and absorb it), a standard AC transformer being a bi-directional device will naturally couple this energy back to its AC power input where it is dissipated back into the power grid. This effect eliminates the requirement for the transformer to absorb this energy. Similarly, when an mains-borne input transient enters into a SMPS, it cannot easily pass this pulse to its outputs and again must attempt to absorb it without exploding. Virtually all AC transformers in an industrial setting have loads such as motors, heaters, lamps which are well able to safely absorbing most transients passed on to them.